1. Field of the Invention
The present invention relates to thermistor apparatuses and, more particularly, to an overcurrent-protection thermistor apparatus for protecting communications equipment such as telephone exchanges from overcurrent and to a manufacturing method thereof.
2. Description of the Related Art
Generally, an overcurrent-protection, positive-characteristics thermistor apparatus has been known. The known apparatus has one case in which two positive-characteristics thermistor devices are housed in order to protect communications equipment such as telephone exchanges from overcurrent, caused by lightning surges, contact with power lines or the like, intruding from communication lines. It is preferable that the difference in resistance between the two positive-characteristics thermistor devices is close to 0 .OMEGA.. This is because resistance matching needs to be maintained between the transmission and receiving circuit lines in communication circuits in communications equipment such as telephone exchanges.
In the conventional positive-characteristics thermistor apparatus, troublesome work has been required to make the difference in resistance between the two positive-characteristics thermistor devices close to 0 .OMEGA.. Selecting and pairing two positive-characteristics thermistor devices having substantially the same resistance among a number of positive-characteristics thermistor devices has been required. This work is made more complicated because positive-characteristics thermistor devices display large variations in resistance due to slight differences in manufacturing conditions.
A method can be considered in which positive-characteristics thermistor devices are classified into groups according to their resistances and then thermistor devices in a certain group are paired. If the resistance of each of the two positive-characteristics thermistor devices is measured at different times, however, the measurement data may not be accurate due to a change in the ambient temperature at each measurement or a minute change due to aging of the resistance measuring instrument, thereby the difference in resistance between the two combined thermistor devices can become large. In the worst case, resistance matching between the transmission and receiving circuit lines cannot be maintained.
Another method can be considered in which the resistance of each positive-characteristics thermistor device is measured and a device having a too-low resistance is trimmed to have a higher resistance such that all the thermistor devices have the specified resistance in the end. If the resistances of the two combined thermistor devices are measured at different times before they are trimmed, the measurement data may not be accurate due to the above-described reasons, making the difference in resistance measurements between the two thermistor devices inaccurate. Therefore, trimming cannot be conducted accurately and the resistance difference between the two thermistor devices can become large.